Process for making molded polymeric product with multipass expansion of polymer bead with low blowing agent content

ABSTRACT

A method for making low density (0.8-1.1 lb/cu. ft.) expanded polymeric products uses blowing agent in an amount of only from 2 to 4.4 weight percent. The process uses 2 to 5 expansion steps together with a polymer having a particular polydispersity, weight average molecular weight and M z  :M n , this polymer having a greater expandability than conventional polymers. The process requires only about half of the amount of blowing agent currently being used in commercially viable processes for making expanded polystyrene products. The process can be used with or without a molding step.

This is a division, of application Ser. No. 07/618,359 filed Nov. 26,1990.

BACKGROUND OF THE INVENTION

The present invention relates to a process for the manufacture ofexpanded polymeric products. The process is directed at the reduction ofthe amount of blowing agent required in order to manufacture a lowdensity foam product. This is achieved by using: (1) a low level ofblowing agent, in combination with both (2) multiple premoldingexpansion steps, as well as (3) a polymer having a certain molecularweight distribution, which polymer is highly expandable. The finalresult is a process which utilizes less blowing agent, resulting in notonly a cost savings, but more importantly, reduced release ofenvironmentally-damaging volatile organic compounds (VOCs) into theatmosphere. Preferably the polymer is polystyrene.

The production of molded polystyrene products has required making anintimate mixture of a polystyrene polymer and a blowing agent. Incommercial operations, this intimate mixture was generally made intosolid (relatively "high-density") beads of relatively small size (e.g.beads having a diameter of from about 0.2 to 4 millimeters). The beadswere then expanded (via heating the mixture) in order to make anexpanded polystyrene product. The expansion was usually carried out byheating the beads above their softening temperature and above theboiling point cf the blowing agent (usually pentane), resulting in thevaporization of the blowing agent. [The blowing agent must have aboiling temperature below the softening temperature of the polymerblowing agent mixture.]The vaporization of the blowing agent caused anexpansion of the beads to form individual particles of foam. Theexpansion was generally carried out by using a first expansion step,whereafter expanded particles (referred to as prepuff) were then aged,and thereafter placed into a mold and again heated, whereby the prepufffurther expanded and, because of the confined volume, fused to form aunitary object. Under optimal conditions, the bonds formed between theindividual prepuff particles were stronger than the individual particlesthemselves. That is, upon stressing the finished, molded object enoughto cause it to break, the break would occur mostly across the individualprepuff particles, rather than at the junctions and interstices of theprepuff particles.

Fundamental to the foaming operation is the requirement that the polymercontain a blowing agent. In prior commercial production, steam has beenused to heat the beads, the steam causing the blowing agent to vaporize,which vaporization results in the formation of gaseous bubbles withinthe polymer. These bubbles expand as the internal pressure increases. Afoam is produced because the bubbles, for the most part, are trapped,resulting in the production of a foam. Since the vast majority of thevolume within the foam is occupied by these bubbles, the resulting foamhas a density much lower than that of the unexpandable polymer/blowingagent mixture. The blowing agent also diffuses out over time.

A large fraction of the foamed polystyrene currently being produced hasa density of from 0.8 lb./cu.ft. to 1.1 lb./cu.ft. This density ofmaterial is generally used for building insulation and/or for protectivepackaging. In order to achieve this density, it has been the practice ofthe commercial manufacturers of such "low-density" expandablepolystyrene to incorporate from about 6 to about 8 weight percent ofpentane into the polystyrene polymer. Beads are formed via a suspensionpolymerization during or after which pentane is introduced into thebeads. These beads are then heated in a single premolding expansion stepin which the beads are expanded in volume by a factor of about 40 (i.e.to a density of about 1.0 lb/cu.ft.). The now preexpanded beads(prepuff) are allowed to cool and equilibrate (i.e. aged) by permittingair to diffuse thereinto, and are then put into a mold where they areagain heated, resulting in the further expansion and fusion of thepreexpanded prepuff, so that the prepuff particles are bonded together.

U.S. Pat. No. 2,884,386 describes a process for making cellular bodiesof organic thermoplastic materials. The process described thereininvolves making an intimate mixture of a blowing agent with athermoplastic resin and thereafter expanding the mixture to form acellular thermoplastic body. The specification refers to the use ofcycles of expanding operations that, if used repeatedly, cause furtherexpansion of the prepuff particles made in accordance therewith.However, the '386 patent nowhere provides any general statement as tohow many premolding expansion steps should be utilized.

The '386 patent nowhere provides any general description of the amountof blowing agent to be used in the process. However, the '386 patentdoes state that the preferred primary blowing agent isdichlorodifluoromethane, and the Examples in the '386 patent all utilizeonly halogenated methanes as the blowing agent. Example 1 of the '386patent utilizes 2 weight percent dichlorodifluoromethane with eightexpansion steps to achieve an undisclosed final product density. Example2 utilizes 8.11 weight percent dichlorodifluoromethane with nineexpansion steps to effect a 150× volumetric increase (the density of thefinal product was undisclosed). Example 4 utilized 20 volume percent ofdichlorodifluoromethane in a three step expansion process, to effect afinal product density of 0.938 lb./cu. ft. In comparison with theprocess of the present invention, these examples (as well as theremaining examples of the '386 patent) utilize such a high level ofblowing agent (and/or such a high number of expansion steps) that theprocess of the present invention is not only not suggested, the processof the present invention is also taught away from.

The '386 patent also fails to provide one with the unexpected result ofthe present invention: i.e. that if one were to use merely from 2 to 4.4weight percent of a blowing agent, a product of relatively low density(i.e. from 0.8 to about 1.1 lb/cu.ft.) could be produced with only from2 to 5 expansion steps. The gist of the '386 patent is the generalizednotion that multiple expansion steps can be used to effectuate avolumetric increase greater than the theoretical volumetric increasepossible from the expansion of the blowing agent alone. As the '386patent states repeatedly, this increase is brought about allowing a morepermeable secondary blowing agent, such as air, to diffuse into the foamwhereupon after cooling an additional heating will produce furtherexpansion due to the presence of this secondary blowing agent in furtherheating/expansion steps. Although the process of the present inventioncertainly utilizes this mechanism of increasing the degree of expansion,the process of the present invention is directed towards a specific areawherein this mechanism is used in addition to other critical processsteps, i.e. the use of a low (2-4.4 weight percent) level of blowingagent, in combination with the use of only 2 to 5 expansion steps, aswell as the use of a specific polymer type (i.e. a polymer exhibitingthree characteristics: (1) a polydispersity of from about 1 to less than2.5; (2) a weight average molecular weight of from greater than 180,000to about 300,000; (3) a Mz:Mn of from about 2 to about 4.5; and (4) isbranched to from 0 to 5 weight percent). It should be noted that the'386 patent nowhere discloses a polymer having such characteristics.

Furthermore, the process of the present invention isolates a specificarea of improvement over the subject matter disclosed in the '386patent. The process of the present invention relates to the use of beadsof thermoplastic polymer which contain only from about 2 weight percentto about 4.4 weight percent of a hydrocarbon blowing agent. It hassurprisingly been found that even with such a small amount of theblowing agent, the bead can be expanded to a final density of from about0.8 to about 1.1 lb./cu. ft., while using only 2 to 5 expansion steps(or 2 to 4 preexpansion steps before the molding step). The '386 patentnowhere achieves such final product densities while utilizing so littleblowing agent.

Applicants have discovered that the use of a low amount of blowing agentprovides many important advantages, among which are:

(1) a reduction in the amount of blowing agent required, resulting incost savings;

(2) reduced environmental pollution since less blowing agent (generallya volatile organic compound, i.e. a VOC) is released into theenvironment both during manufacture and during consumer use;

(3) processing advantages such as:

(a) a lower shrinkage in the molding step;

(b) quicker cooling in the molding step, resulting in shorter processingtimes;

(c) shorter aging time between the preexpansion steps and between thelast preexpansion step and the molding step, resulting in shorterprocessing times; and

(d) ability to be easily molded with acceptable fusion and dimensionalstability on molding while using a low level of blowing agent.

U.S. Pat. No. 4,839,396 describes a process for making expandablealkenyl aromatic polymer particles. These particles have the ability touse a decreased amount of blowing agent while maintaining the potentialto produce a bulk density equivalent to that achieved by particlescomprising a greater amount of blowing agent. This is achieved throughthe use of from 0.005 to 0.5 weight percent of a "density modifier". The'396 patent describes the density modifier as a compound providingthermal stability for the alkenyl aromatic polymer at extrusion andexpansion conditions and which is also a liquid plasticizer at expansionconditions. These density modifiers are stated to include octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate, as well as ethylenebis(oxyethylene)bis(3-tertbutyl-4-hydroxy-5-methylhydrocinnamate).

The specification of the '396 patent states that the "volatile fluidfoaming agents" (i.e. the blowing agents) usually are employed inamounts corresponding to from about 5 to about 15 percent of the weightof the total formulation. The only examples in the '396 patent utilizefrom 8.8 to 10.3 weight percent of the blowing agent. These examplesshow that the amount of the blowing agent was reduced from a level of10.0-10.3 weight percent down to about 8.8 weight percent (i.e. areduction of about 12 to 15 percent in the amount of blowing agentused), while achieving the same final density as in the comparative runhaving the greater amount (i.e. 10.0-10.3 weight percent) of blowingagent present.

The process of the present invention also has as a goal the reduction inthe amount of blowing agent used in the manufacture of an expandedpolymer. However, relative to the '396 patent, the process of thepresent invention permits at least as much as twice the percentagereduction in the amount of blowing agent (e.g. most preferably areduction of from about 6 weight percent to about 3.5 weight percent,which is approximately a 40 percent reduction). The process of thepresent invention produces this comparatively large reduction in blowingagent via an approach which is different from the approach taken in the'396 patent. This approach is the use of an initially low level of theblowing agent (i.e. a blowing agent level of from 2 to 4.4 weightpercent) while simultaneously using multiple preexpansion steps. Itshould be noted that although the '396 patent suggests the use ofmultiple preexpansion steps before the molding step, the '396 patentfails to make any connection between the use of low amounts of blowingagent and the use of multiple preexpansion steps, and the '396 patenthas a very broad, undirected disclosure of the amount of blowing agentwhich can be used. Furthermore, the '396 patent suggests the use of onlyrelatively high amounts of blowing agent (i.e. 5 to 15% broadly, withexamples limited to from 8.8% to 10.3% by weight).

U.S. Pat. No. 4,520,135 and 4,525,484 (a divisional of the applicationfiled for the '135 patent) are both directed at a polystyrene particlescontaining a blowing agent, wherein the polystyrene has an improvedexpandability. More particularly, the polystyrene particles arecomprised of a polymer which has a molecular weight of from about130,000 to about 180,000. The '484 patent describes several methods formaking this polymer, i.e. via the use of chain transfer agents, the useof oligomers, or polymerizing in the presence of the blowing agent. The'484 patent states that the resulting polystyrene particles can beexpanded by conventional methods (e.g. steam expansion). The '484 patentstates that the blowing agent can be present in an amount generally from3 to 12 weight percent (preferably from 5 to 8 weight percent, and theonly example in the '484 patent utilizes approximately 7 weight percentpentane). However, the '484 patent nowhere makes any statement regardingthe use of multiple remolding expansion steps. In contrast, the processof the present invention utilizes a polymer which is different from thepolymer described in the '484 patent in that it exhibits threecharacteristics which are different from the polymer described in the'484 patent. Foremost among these differences is the fact that theweight average molecular weight of the polymer used in the process ofthe present invention is higher than that of the polymer described inthe '484 patent. Surprisingly, this higher molecular weight polymer hasa degree of expandability at least as high as the polymer described inthe '484 patent. In further contrast, the process of the presentinvention requires the use of from 2 to 4 premolding expansion steps,whereas the '484 patent makes no mention of multiple expansion stepseither with or without molding.

U.S. Pat. No. 4,485,193 describes a process for making resilient foamparticles and moldings with a "lightly crosslinked" polymer, which couldbe a styrene polymer. The process entails the use of a volatile fluidfoaming agent that has low permeability through the polymer, and theprocess also uses multiple expansion steps for the production of foamsof low density, i.e. suitable for molding. The process described andclaimed in the '193 patent requires that the once-expanded particles aresubjected to a superatmospheric pressure of at least 3 atmospheres inair, whereafter the now "pressurized" particles are further expanded byheating the particles above the glass transition temperature of thepolymer after the particles are returned to a normal atmosphericpressure.

The '193 patent nowhere provides any generalized description of theamount of blowing agent which may be employed in the process describedtherein. However, of the 43 Samples discussed in the '193 patent:Samples 1-17 contained blowing agent in an amount of from about 20 toabout 30 weight percent; Samples 18-21 were foams made "in accord" withthe specification of the '193 patent but no data was provided re theamount of blowing agent employed in the making of the foam; Sample 22was a foam made in accord with the conditions of Sample 2 (and Sample 2utilized 28.9 weight percent blowing agent); Samples 23-38 containedblowing agent in an amount of from 6.12 to about 7.0 weight percent;Samples 39-41 do not contain any description of the amount of blowingagent, but merely state that blowing agent was permitted to diffuse intothe already-formed beads; and Samples 42 and 43 appear to utilizeblowing agent in an amount of at least 11 weight percent. In summary,the '193 patent teaches the use of blowing agent in an amount which isconsiderably higher than the amounts involved in the present invention.

U.S. Pat. No. 3,639,551 describes a cyclic method for producinglow-density polystyrene foam beads, wherein the beads are expanded in aplurality of expansion steps. However, the gist of the '551 patent isthat in between the expansion steps the now partially-expanded beads arereheated to restore the majority of the "lost volume" (i.e. the volumelost upon the cooling of the beads immediately after they wereexpanded). This reheating step precedes the next expansion step. By thismethod, the shrinkage of the beads will be prevented from substantiallyaffecting the ultimate degree of expansion obtained.

The '551 patent nowhere provides any discussion of the quantity ofblowing agent to be employed in expanding the beads. The '551 patent hasa single example which states that:

1000 pounds of a commercial grade acrylonitrile styrene copolymer beadshaving low boiling hydrocarbon propellant (pentane) included therein anda diameter between about 1/64 to about 1/32 inch were stored in a hopperand fed into a "Buccaneer" preexpander (available from TRI Manufacturing& Sales Co., Lebanon, Ohio) having an expansion chamber substantially asdescribed above.

The '551 patent nowhere states how much of the low boiling propellant(pentane) was present in the beads.

In stark contrast to the '551 patent, the process of the presentinvention utilizes a low level of blowing agent (from about 2 to about4.4 weight percent) in combination with multipass expansion in order toachieve a product having a density of from about 0.8 to 1.1 pounds percubic foot. The '551 patent nowhere mentions the use of such anunconventionally low amount of blowing agent. The process of the presentinvention further requires the use of a specific polymer, which polymerthe '551 patent nowhere discloses.

U.S. Pat. No. 3,631,133 describes a process for expanding polystyrenebeads in order to produce a bead having an exceptionally low density(i.e. 5 kg./cu. meter, or less, which equals approximately 0.3 lb./cu.ft., or less). The method described in the '133 patent is generallydescribed as:

(1) insufflating (i.e. pre-expanding to produce a partly expandedproduct) polystyrene granules containing blowing agent, the insufflatingbeing carried out with steam at about atmospheric pressure, whereby thegranules are partially expanded;

(2) conditioning the partially expanded granules at atmosphericpressure;

(3) subjecting the partially expanded granules (in a confined space) tosteam at about I50 g./sq. cm. pressure; and

(4) restoring the expanded granules to atmospheric temperature andpressure.

The gist of the '133 patent is that of using multiple expansion steps incombination with a conditioning step, in order to ultimately produce alow density product. The '133 patent nowhere refers to the quantity ofblowing agent to be utilized in the process. Rather, all thespecification (including Examples) has to say about the blowing agentis:

The starting material consisting of granules of polystyrene containing(sic) a pentane petroleum fraction as a blowing agent. [Col. 2, lines61-63]

In stark contrast to the '133 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 1.1 pounds per cubic foot. The '133 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '133 patent nowhere refers to the characteristics ofthe polystyrene polymer used therein as providing anything other than aconventional level of expandability. U.S. Pat. No. 3,598,769 describes aprocess for expanding polystyrene, this process involving:

(1) subjecting (for a few minutes) polystyrene granules to steam at lowpressure;

(2) conditioning the granules for a few hours at about 20° C. to 40° C.;

(3) reheating the expanded granules to about 100° C. with hot air;

(4) then treating the granules with steam for 30 to 40 seconds; followedby

(5) conditioning the granules for 1 to 24 hours.

The objective of the '769 patent is to provide a process for producingbeads of polystyrene having an apparent specific mass less than about 7kg./cu. meter (i.e. a density of about 0.44 lb/cu.ft., or less). Thegist of the '769 patent is to provide a very specific process for usingtwo of expansion steps and a conditioning step after each expansionstep. Furthermore, the '769 patent is directed at carrying out thisprocess on a continuous conveyor belt.

As with the '133 patent, the '769 patent nowhere describes the amount ofblowing agent to be used in the process.

In stark contrast to the '769 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 1.1 pounds per cubic foot. The '769 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '769 patent nowhere mentions the use of a polymerhaving an extraordinary degree of expandability.

U.S. Pat. No. 3,126,432 describes a process for producing super-lowdensity thermoplastic foam, namely polystyrene foam. The processdescribed in the '432 patent involves expanding particles of polystyrenehaving a vaporizable liquid (butane or pentane) inflating agent therein,and thereafter aging the expanded particles first at atmosphericpressure and thereafter at superatmospheric air pressure (2 to 8atmospheres) for several hours. This exposure to superatmospheric airpressure has the effect of causing a secondary blowing agent to migrateinto the expanded particles. Thereafter, the pressure is released andwithin five hours the particles are heated in a closed mold. Thus thegist of the '432 patent is to "pump up" the expanded particles byexposing the particles to superatmospheric air, and thereafter carryingout a second expansion step by taking advantage of the relatively highinternal pressure within the particles, once they are released from thepressure chamber.

The '432 patent nowhere has an general discussion of the amount ofblowing agent to be utilized in making polystyrene foams. Of the fiveexamples given in the '432 patent, only Examples I, III, and V provideany information as to the amount of blowing agent used in the process.In each of these Examples, the blowing agent used is pentane, and thepentane is present in an amount of 6% by weight of the polystyreneglobules. Thus it is clear that the '432 patent does not teach towardsany process which utilizes a blowing agent in an amount less than 6% byweight.

In stark contrast to the '432 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 1.1 pounds per cubic foot. The '432 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '462 patent nowhere mentions a polymer exhibiting theproperties of the present invention.

U.S. Pat. No. 3,056,753 describes the production of expandable polymericparticles having a foamed polymeric structure. The process describedtherein involves:

(1) partially expanding the polymeric particles; followed by

(2) crushing the particles; followed by

(3) again partially expanding the particles.

The gist of the '753 patent is to decrease the molding cycle time,whereby molded articles can be removed from the mold after permitting acooling period of lower duration. Nowhere in the '753 patent is thereany mention of the amount of blowing agent to be utilized in theprocess. In fact, even the seven examples within the '753 patent fail toprovide any information as to the amount of blowing agent utilized.

In stark contrast to the '753 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 1.1 pounds per cubic foot. The '753 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '753 patent nowhere mentions a polymer having thecharacteristics of the polymer of the present invention.

U.S. Pat. No. 4,721,588 describes a closed circuit process for theproduction of expanded polystyrene foam. This process comprises thesteps of:

(a) pre-expanding raw polystyrene beads containing a blowing agent in apre-expansion vessel;

(b) storing the beads in one or more closed storage containers to allowthe internal pressure within the expanded beads to return tosubstantially atmospheric pressure;

(c) molding the expanded beads to a desired configuration in a closedmold with steam; and

(d) removing the thus-formed article from the mold and placing such inan aging room, wherein at each stage the blowing agent released from thebeads is recovered, separated from any residual steam by means of acondensing system, and introduced into the burner of a steam generator,thereby serving as fuel for the process.

As can be seen from the above description of steps, the gist of theprocess described in the '588 patent is the recovery of the blowingagent and its re-use as a fuel for the heating step. This produces thedual effects of (I) reducing the amount of volatile organic compoundsreleased into the atmosphere, as well as (2) obtaining a double use forthe blowing agent which escapes from the polystyrene during thepre-expansion and molding steps.

The '588 patent also mentions that the blowing agent can be n-pentane,or mixtures of n- and iso-pentane (up to about 25% isopentane byweight). The '588 patent also states that the initial blowing agentcontent of the expandable polystyrene beads can be 4-8 weight percent.However, the '588 patent nowhere states that a low density foamedpolystyrene product can be obtained if one utilizes less than 5 weightpercent of the blowing agent. In fact, aside from the statement that ablowing agent content of 4-8 weight percent can be used, the '588 patentmakes absolutely no mention of actual product densities or any furthermention of the amount of blowing agent in any actual composition.Finally, the '588 patent makes no mention of the use of multiplepre-molding expansion steps. Rather, the '588 patent teaches one tosimply perform one pre-molding expansion step and to thereafter followthis step with the molding step.

In contrast to the '588 patent, the process of the present inventionutilizes a low level of blowing agent in combination with multipassexpansion in order to achieve a product having a density of from 0.8 to1.1 pounds per cubic foot. The '588 patent nowhere mentions the use ofthe combination of a level of blowing agent less than 5 weight percentwith multipass expansion in order to achieve a product density of from0.8 to 1.1 pounds per cubic foot. Furthermore, the '588 patent nowherementions a polymer having the characteristics of the polymer used in theprocess of the present invention.

In recent years the emissions of volatile organic compounds (i.e. VOC's)have come under increasing scrutiny by the EPA, state and local airquality boards as mandated by the Clean Air Act of 1977. Becausehydrocarbon emissions have been shown to contribute to photochemicalsmog, the expanded polystyrene industry which uses pentane as a blowingagent has come under pressure to limit its use and/or emissions ofpentane.

Since the early months of 1990, the inventors' process has enjoyed ahigh level of commercial success, with sales of at least 3 millionpounds of a formulation which has been expanded to 0.8-1.1 lb./cu. ft.only with the inventor's process, which formulation has ahighly-expandable expandable polymer present in an amount of about 96weight percent, based on the total weight the formulation. Thus therehas been a high level of commercial success of both the process as wellas the formulation utilized in practicing the process.

For several years BASF Corporation has been involved in the manufactureand sale of a number of expandable polystyrene formulations havingapproximately 6 weight percent pentane therein. Typically theseformulations contained a polymer having a polydispersity of 2.2, aweight average molecular weight of about 190,000, and an Mz:Mn of about3.5. In stark contrast, the product of the present invention has apolydispersity of from 1 to less than 2, a weight average molecularweight of from about 200,000 to about 300,000, and an Mz:Mn of fromabout 2 to less than 3.

One polymer which has been commercialized for several years has apolydispersity of about 1.9, a weight average molecular weight of about190,000, and furthermore, upon analysis, yielded a ratio of M₂ to M_(n)of 3.04. Furthermore, this polymer was produced only in formulationsbearing blowing agent in an amount of about 6 weight percent. Incontrast, the polymer of the present invention has a combination ofcharacteristics (polydispersity, weight average molecular weight, andM_(z) :M_(n)) which differs from the aforementioned commerciallyavailable polymer. Furthermore, the formulation of the present inventionutilizes blowing agent in an amount of only from about 2 weight percentto about 4.4 weight percent.

BRIEF SUMMARY OF THE INVENTION

The process of the present invention pertains to making a closedcellfoamed thermoplastic resinous object. The process is carried out byexpanding beads in from 2 to 5 expansion steps. The beads are comprisedof a blowing agent and a polymer. The blowing agent is homogeneouslydispersed in the polymer and the blowing agent may be, in general,hydrocarbons which are gaseous or liquid at standard temperature andpressure, do not dissolve the styrene polymer, and boil below thesoftening point of the polymer. The blowing agent is preferably at leastone member selected from the group consisting of:

pentane, cyclopentane, neopentane, isopentane, pentane petroleumdistillate fractions, propane, butane, isobutane, hexane, isomers ofhexane, 2-methyl pentane, 3-methyl pentane, methylcyclopentane,cyclohexane, methylcyclohexane, heptane, propylene, 1-butylene,2-butylene, isobutylene, mixtures of one or more aliphatic hydrocarbonshaving a molecular weight of at least 42 and a boiling point not higherthan 95.C at 760 millimeters absolute pressure, water, carbon dioxide,ammonium carbonate, and azo compounds that are decomposable to form agas at a heat-plastifying temperature to which the polymer is brought.

The blowing agent is present in the beads in an amount of from about 2weight percent to about 4.4 weight percent based on the weight of thebeads.

The polymer making up the beads may be one or more polymers producedfrom at least one of a variety of monomers. The monomer is at least onemember selected from the group consisting of:

styrene, derivatives of styrene, vinyltoluene, mono- and polyhalogenatedvinyltoluenes which form linear polymers, acrylonitrile, and methylmethacrylate.

The polymer is present in the beads in an amount of from about 93 weightpercent to about 98 weight percent based on the weight of the beads. Thepolymer exhibits the following three characteristics: (1) apolydispersity of from about 1 to less than 2.5; (2) a weight averagemolecular weight of from greater than about 180,000 to about 300,000;and (3) an Mz:Mn of from about 2 to about 4.5. Furthermore, the polymeris branched to from 0 to less than 5 weight percent.

The expansion steps are carried out in an expander at substantiallyatmospheric pressure. The expansion steps result in "finally-expanded"beads. The expansion of the polystyrene beads is carried out in a mannerso that the finally-expanded beads have a density of from about 0.8pounds per cubic foot to about 1.1 pounds per cubic foot. The process ofthe present invention also pertains to a process for making aclosed-cell foamed thermoplastic resinous molded object. The process iscarried out by expanding beads in from 2 to 4 preexpansion steps, andthereafter carrying jout a molding step in which the beads are furtherexpanded and fused into a unitary object. In the process in which thereis a molding step, the expandion steps which precede the molding stepare termed "preexpansion" steps because they precede the molding step.It should be noted that the molding step generally causes at least somefurther expansion, along with bonding the preexpanded beads to oneanother (i.e. fusion).

The beads are comprised of a blowing agent and a polymer. Although theblowing agent may generally be as described above, preferably theblowing agent is at least one member selected from the group consistingof:

pentane, cyclopentane, neopentane, isopentane, pentane petroleumdistillate fractions, propane, butane, isobutane, hexane, isomers ofhexane, 2-methyl pentane, 3-methyl pentane, methylcyclopentane,cyclohexane, methylcyclohexane, heptane, propylene, 1-butylene,2-butylene, isobutylene, mixtures of one or more aliphatic hydrocarbonshaving a molecular weight of at least 42 and a boiling point not higherthan 95.C at 760 millimeters absolute pressure, water, carbon dioxide,ammonium carbonate, and azo compounds that are decomposable to form agas at a heat-plastifying temperature to which the polymer is brought.

The blowing agent is present in the beads in an amount of from about 2weight percent to about 4.4 weight percent based on the weight of thebeads.

The polymer making up the beads may be one or more polymers producedfrom at least one of a variety of monomers. The monomer is at least onemember selected from the group consisting of:

styrene, derivatives of styrene, vinyltoluene, mono- and polyhalogenatedvinyltoluenes which form linear polymers, acrylonitrile, and methylmethacrylate.

The polymer is present in the beads in an amount of from about 93 weightpercent to about 98 weight percent based on the weight of the beads. Thepolymer exhibits the following three characteristics: (1) apolydispersity of from about 1 to less than 2.5; (2) a weight averagemolecular weight of from greater than about 180,000 to about 300,000;and (3) an Mz:Mn of from about 2 to about 4.5. Furthermore the polymeris branched to from 0 to less than 5 weight percent.

The preexpansion steps are carried out in an expander at substantiallyatmospheric pressure. The completion of the preexpansion steps resultsin the production of "finally-preexpanded" beads. Thefinally-preexpanded beads are then molded in order to further expand andfuse the finally-preexpanded beads. Both the preexpansion steps and themolding step are carried out so that a molded foamed object having adensity of from about 0.8 to about 1.1 pounds per cubic foot is formed.

It is an object of the present invention to reduce the amount of blowingagent used in the production of low density, closed cell, foamedthermoplastic resinous objects.

It is an object of the present invention to reduce the level ofenvironmental impact (i.e. reduced VOC emissions) in the production oflow density, closed cell, foamed thermoplastic resinous objects.

It is an object of the present invention to provide a process which willlower the shrinkage upon molding in the production of closed cell,foamed thermoplastic resinous objects.

It is an object of the present invention to provide a process whichresults in a lowering of the required cooling times both betweenexpansion (and preexpansion) steps as well as in any molding step whichis utilized.

It is an object of the present invention to provide a process forefficiently using a polystyrene polymer having a high degree ofexpandability.

It is a further object of the present invention to provide a process forutilizing an expandable polystyrene formulation in the production ofexpanded polystyrene products.

It is a further object of the present invention to provide a processwherein a polystyrene polymer as well as a formulation for makingexpanded polystyrene products can be utilized with a lesser amount ofblowing agent emitted, so that there is less blowing agent emitted intothe atmosphere and/or pollution abatement equipment.

It is a further object of the present invention to provide a process forproducing expanded polystyrene products while using a formulation havinga greater ratio of resin to blowing agent, so that more resin is presentper pound of formulation.

It is a further object of the present invention to enable the productionof expanded polystyrene products using decreased molding cycle times, aswell as decreased shrinkage upon molding, as well as decreased agingtimes between expansion steps.

It is a further object of the present invention to enable the productionof an expanded polystyrene product having decreased susceptibility todamage during processing.

It is a further object of the present invention to enable the productionof an expanded polystyrene beads having increased shelf life before themolding step due to a lower rate of loss of blowing agent therefrom.

It is a further object of the present invention to enable a process formaking expanded polystyrene products in which there is decreasedsensitivity to steam during the expansion and molding steps, therebypermitting a "broader molding range process" with respect to the use ofsteam in the preexpansion and molding steps and its effect on fusion,cycle time, and dimensional stability upon molding.

Each of the above objects can further be understood as providing arespective advantage to the process of the present invention. Althoughthe term "polystyrene" is found in the above object statements, theseobjects should be understood as being applicable to all polymers whichmay be utilized in the process of the present invention, as well as apreferred applicability to polystyrene.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention involves expanding a substantiallysolid thermoplastic polymer to form a foam. The expansion of the polymeris effectuated by intimately mixing a blowing agent with the polymer,and thereafter heating the mixture so that the blowing agent vaporizeswithin the polymer particles, causing the polymer particles to expandduring a period in which the polymer is in a softened state. Thevaporization of the blowing agent is produced by the application ofheat. Likewise, the heat also softens the polymer. Enough heat must beapplied to cause the temperature of the polymer to exceed its softeningpoint. The vaporization of the blowing agent within the softened polymercauses the mixture to expand and form a foam. The foam is then allowedto cool, while remaining substantially expanded.

During cooling, the pressure within the foam cells decreases due tocooling and condensing of the blowing agent. This causes gases which canpermeate the polymer (e.g. air, steam, etc.) to migrate into the cells,thereby somewhat restoring (i.e. to atmospheric pressure) the relativelylow internal pressure within the cells. Although components within theatmosphere (i.e. oxygen, carbon dioxide, nitrogen, etc) are to somedegree able to diffuse into the cells, if steam is used as the source ofheat for the expansion steps (or the molding step), it generally is themost permeable of the gases diffusing into the cells of the foam. Uponsubstantial equilibration (i.e. when the pressure within the cells issubstantially that of ambient atmospheric pressure) of the foam, the nowcooled foam can again be heated, resulting in further expansion of thefoam. Thus by utilizing multiple "cycles", or "passes" of suchexpansion, cooling, and "aging" (i.e. substantial equilibration ofpressure), sequential volumetric increases can be achieved.

Optionally, the foam can be further expanded and fused in order to forma molded object. Molding is effectuated by placing preexpanded beadsinto a mold, closing the mold so that a substantially confined volume isproduced, and thereafter further heating the preexpanded beads so thatthey further expand and substantially fill the volume within the moldand fuse (i.e. bond) to one another.

In the process of the present invention it is an objective to provide acommercially viable process which reduces the emission of volatileorganic compounds (VOCs) in comparison with currently viable commercialprocesses. In part, this objective is achieved by utilizing a loweramount of blowing agent than has been used in prior art commerciallyviable processes. This is effectuated by using from 2 to 4.4 weightpercent of the blowing agent, based on the weight of the polymer.

Any one or more of a wide variety of blowing agents can be utilized inthe process of the present invention. These blowing agents include:hydrocarbons which are gaseous or liquid under normal conditions, do notdissolve the styrene polymer, and boil below the softening point of thepolymer. Among the blowing agents preferred for use in the process are,for example: pentane (including isomers of pentane such as cyclopentane,methylcyclopentane, neopentane, isopentane, as well as pentane petroleumdistillate fractions), propane, butane, isobutane, hexane, isomers ofhexane, 2-methyl pentane, 3-methyl pentane, 2,2-dimethylbutane,2,3-dimethylbutane, methylcyclopentane, cyclohexane, heptane, propylene,1-butylene, 2-butylene, isobutylene, mixtures of one or more aliphatichydrocarbons having a molecular weight of at least 42 and a boilingpoint not higher than 95. C at 760 millimeters absolute pressure, water,carbon dioxide, ammonium carbonate, and azo compounds that aredecomposable to form a gas at a heat-plastifying temperature to whichthe polymer is brought. A more preferred group of blowing agentscomprises pentane, cyclopentane, methylcyclopentane, neopentane,isopentane, pentane petroleum distillate fractions, 2-methyl pentane,3-methyl pentane, propane, butane, isobutane, isobutylene, hexane,isomers of hexane, cyclohexane, and heptane.

The blowing agent may be incorporated into the polymer before, during,or after polymerization. In the process of the present invention, theblowing agent is incorporated into the polymer in an amount of from 2 to4.4 weight percent, based on the total bead weight. Preferably theblowing agent is incorporated in an amount of from about 2.5 to about4.4 weight percent, still more preferably from about 3 to about 4, andmost preferably the blowing agent is incorporated in an amount of about3.5 weight percent. Most preferably the blowing agent is pentane (eithern-pentane or a mixture of n-pentane together with isomers of pentane).Preferably the pentane is added to the polymer during the polymerizationprocess, most preferably at a styrene conversion of from 20-60%.

The process of the present invention is carried out on an unexpandedbead. The term "unexpanded bead" is herein defined as a discreteparticle which is comprised of a polymer and a blowing agent.

According to the process of the present invention, unexpanded beads areexpanded in from 2 to 5 expansion steps. The preferred process of thepresent invention involves carrying out 2 or 3 preexpansion steps,followed by a molding step. That is, from 2 to 3 premolding expansionsteps are utilized to produce finally-preexpanded beads. Most preferablythis process utilizes only 2 preexpansion steps.

If the process is being utilized to make a non-molded product, thepreferred process is to use from 2 to 3 expansion steps to producefinally-expanded beads. Most preferably this process utilizes only 2expansion steps.

If the beads are expanded and not fused into a molded object, the beadsare herein termed as "finally expanded beads". If, however, the beadsare ultimately to be fused into a molded object, the expansion steps aretermed "preexpansion steps". Once these beads are "finally preexpanded",they are thereafter further expanded and fused into a molded object in amolding step which also serves as a final "expansion" step. Regardlessof whether the beads are expanded or preexpanded, according to theprocess of the present invention the total number of expansion(including preexpansion) steps is no greater than 5 and no less than 2.It has surprisingly been found that with as little as from 2 to 4.4weight percent of blowing agent, a final product density as low as from0.8 to 1.1 lb/cu.ft. can be obtained. Foams having this density areuseful as insulation and/or for protective packaging.

The expansion of the beads is typically carried out in a batch expanderclosed vessel having steam injected thereinto. Examples of suchexpanders include: Tri 502, Tri 905, Weiser VN400, Kurtz KV1000,Dingledein VA2000. The expansion of the beads is carried out by passingthe beads through an expander so that the beads are heated and becomesoft enough that they expand due to the rising pressure produced by thevaporization of the blowing agent and other internal gases. As a generalrule, the rate of passage of the beads through the expander determinesthe amount of expansion which will result during that expansion step. Ofcourse, the lower the rate of passage of beads through the expander, thegreater the amount of heat transferred to the beads, and the higher theresulting degree of expansion produced. However, there is a maximumamount of expansion which any one expansion step can produce for anygiven bead composition. Thus, it has been found that in general the flowrate of the beads through the expander should be pounds per hour percubic foot of expander volume (i.e. lb/hr./cu.ft.) from about 5 to about120 lb/hr/cu.ft. At flow rates below about 5 lb/hr/cu.ft., the beadsremain in the expander so long that lumping accurrs, and processing timeis uneconomical. At flow rates above 120 lb/hr/cu.ft., the beads do notremain in the expander long enough to cause sufficient expansion toresult in a process of expanding with a reasonable degree of efficiency.Preferably, the expansion rate is from about 7 to about 100lb/hr/cu.ft., and most preferably the expansion rate is from about 12 toabout 80 lb/hr/cu.ft.

The process of the present invention can be carried out either with orwithout a molding step. The following is a description of the moldingstep which can be utilized in the process:

The prepuff (i.e. the preexpanded beads) were placed into a Kurtz vacuumblock mold of internal dimensions of approximately 48"× 96" × 33". Themolding steps were as follows: presteaming vacuum to approximately 0.5bar absolute pressure, followed by steaming into vacuum forapproximately 3 seconds, then cross-steaming through the block for about3-6 seconds, then autoclaving for another 3-8 seconds to a maximum foampressure of approximately 0.5-1.0 bar. Vacuum was then applied to theblock in the mold to assist in cooling the block to a foam pressure ofapproximately 0-0.1 bar, allowing the block to be removed from the moldwithout significant post-expansion or shrinkage occurring.

The polystyrene beads can also contain other additives which impartparticular properties to the expandable products, such as antistaticagents, stabilizers, colorants, lubricants, fillers, substances whichprevent agglomeration during prefoaming, e.g. zinc stearate,melamineformaldehyde condensates or silica, and agents for reducing thedemolding time during final foaming, e.g. glycerol esters orhydroxycarboxylic acid esters. Depending on their intended effect, theadditives may be homogeneously dispersed in the particles or be presentas a surface coating.

A total of about 0.7 parts (based on 100 parts styrene) of free radicalinitiators are added to the organic phase at the beginning of thepolymerization reaction. Preferably the initiator is a mixture ofbenzoyl peroxide, dicumyl peroxide, t-butyl perbenzoate. While thebenzoyl peroxide acts as a "low temperature" initiator, the t-butylbenzoate acts as a "high temperature" initiator. Furthermore, any excessdicumyl peroxide (which is left after the polymerization reaction iscomplete) serves as a synergist for flame retardant properties.

The process of the present invention also may optionally utilizepoly-n-vinylpyrollidone as a protective colloid which coats the beads.The poly-n-vinylpyrollidone is added at a time during the polymerizationreaction when the beads are at the size desired. Thepoly-nvinylpyrollidone has the effect of coating the beads so that theycannot adhere to one another, resulting in arresting the growth of thebeads, thereby "freezing" their size. The poly-n-vinylpyrollidone ispreferably added to the reaction mixture at a level of about 0.3 weightpercent, based on the weight of the polymer present.

Generally, the polymers to be used in the process of the presentinvention may be produced from one or more of any of a wide variety ofmonomers. Such monomers include monovinyl compounds which undergoaddition polymerization to provide generally linear polymers. It ispreferred that such polymers are also capable of forming structurescrosslinked to a desired degree when polymerized in the presence of acrosslinking quantity of a polyvinyl compound (e.g. ethylene glycol,dimethacrylate, divinylbenzene, etc.). The monovinyl compounds useful inthe process are, for example, styrene (and derivatives thereof),vinyltoluene, mono- and polyhalogenated vinyltoluenes which form linearpolymers, acrylonitrile, methyl methacrylate, and vinyl toluene.

Preferably the monomer is at least one member selected from the groupconsisting of styrene and derivatives of styrene. Polystyrene is themost preferred polymer for the process of the present invention.Although pure polystyrene is the most preferred styrene polymer for usein the present invention, monomers herein termed "derivatives ofstyrene" which can be polymerized and used in the process of the presentinvention include:

alpha-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene,ar-ethylstyrene, ar-vinylxylene, ar-chlorostyrene, and ar-bromostyrene,or solid copolymers of two or more of such alkenyl aromatic compoundswith minor amounts of other readily polymerizable olefinic compoundssuch as divinylbenzene, methylmethacrylate or acrylonitrile, etc.

The process of the present invention requires the use of a polymer whichexhibits the following three characteristics: (1) a polydispersitywithin a given range; (2) a weight average molecular weight within agiven range; and (3) an M_(z) :M_(n) within a given range. Furthermore,the polymer used in the process of the present invention is hereindefined in terms of weight average molecular weight (M_(w)), numberaverage molecular weight (M_(n)), z-average molecular weight (M_(z)).The number average molecular weight is the arithmetic mean valueobtained by diving the sum of the molecular weight by the number ofmolecules. The weight average molecular weight is the second poweraverage molecular weight in the polydisperse polymer. The z-averagemolecular weight molecular weight is the third power average molecularweight in the polydisperse polymer. More extensive and descriptivedefinitions of these various molecular weights were described byBillmeyer, F.W., Jr., Textbook of Polymer Science, 2nd Ed.,1971,Wiley-Interscience, N.Y., N.Y., pp 6, 66, 78, and 92, which book isherein incorporated by reference.

The inventors of the process of the present invention have unexpectedlydiscovered that such a polymer can also be used to make a low density(i.e. 0.8-1.1 lb/cu.ft.) foam while using an unexpectedly low amount ofblowing agent, if from just two to five expansion steps are utilized inthe process.

The process of the present invention utilizes a polymer having aparticular set of characteristics, which characteristics are derivedfrom the molecular weight distribution curve of the polymer. Themolecular weight distribution curve is determined by gel permeationchromatography. This method is described in detail in G. Glockler,Polymercharakterisierung, Chromatographische Methoden, volume 17,published by Huthig, Heidelberg 1982, which is herein incorporated byreference.

The first of these characteristics, i.e. polydispersity, is determinedby analyzing the molecular weight distribution curve for the reactionproduct of the polymerization. Polydispersity is calculated by dividingthe weight average molecular weight by the number average molecularweight. Thus, the polydispersity is a measure of the breadth of themolecular weight distribution. The polymer generally exhibits apolydispersity of from about 1 to less than 2.5. Preferably, the polymerexhibits a polydispersity of from about 1 to less than 2.0, still morepreferably from about 1.5 to less than 2.0, and most preferably thepolymer exhibits a polydispersity of from about 1.7 to about 1.98.Example 7 (infra) discribes the method of analysis of the polymericreaction product, this method providing the means for determination ofweight average molecular weight, number average molecular weight, and"z-average molecular weight". Thus this analytical procedure providesthe data from which one may then calculate polydispersity, weightaverage molecular weight, and the M_(z) :M_(n) ratio.

The second characteristic which the polymer exhibits (i.e. the weightaverage molecular weight) is, in general, from greater than about180,000 to about 300,000. Preferably, the polymer used in the process ofthe present invention has a weight average molecular weight of fromgreater than 190,000 to about 250,000. Most preferably, the polymerexhibits a weight average molecular weight of from about 200,000 toabout 220,000. As with polydispersity, the weight average molecular isdetermined the analysis provided in Example 7, infra.

The third characteristic is M_(z) :M_(n), i.e. the ratio of thez-average molecular weight to the number average molecular weight. Thisratio is related to the steepness of slope of the upper end of themolecular distribution curve. In general, the polymer exhibits an M_(z):M_(n) ratio of from about to 2.5 to about 3.3. Most preferably thepolymer exhibits an M_(z) :M_(n) ratio of from about 2.7 to about 3.0.As with polysidpersity and weight average molecular weight, the M_(z):M_(n) ratio can be calculated based upon the analytic results obtainedfrom the procedure of Example 7. This procedure, of course, results inobtaining a molecular weight distribution curve. The value for weightaverage molecular weight, number average molecular weight, and"z-average molecular weight" can be determined. These values permit thecalculation of polydispersity as well as M_(z) :M_(n) ratio.

The polymer used in the process of the present invention is asubstantially linear polymer, i.e. is a substantially unbranchedpolymer. In general the polymer has a degree of branching of from 0 toless than 5 weight percent. The phrase ". . . branched to from 0 to lessthan 5 weight percent . . . " is herein defined as referring to apolymeric chain in which at least 95 percent of the molecular weight ofthe polymer resides in that portion of the molecule which constitutesthe linear chain. For purposes of calculating the weight percent of thepolymer which resides in branches (as opposed to the linear portion ofthe polymer molecule), carbon atoms which are not part of the mainpolymeric chain are considered to be located on branches, and any atomswhich are attached to the branch carbon atoms are likewise considered tobe located on the branch portion of the polymer molecule. Non-carbonatoms which are bonded to a carbon atom of the linear polymer backbone(but which themselves do not make up a portion of the backbone) areconsidered substituents rather than branches. However, if a substituentatom is bonded directly or indirectly to a second carbon atom whereinthe second carbon atom is not part of the linear polymer backbone, thesubstituent as well as any atoms attached thereto (which are not part ofthe polymer backbone) are considered to be on a branch. The polymer ispreferably branched to from 0 to less than 2 weight percent. Mostpreferably the polymer is branched to from 0 to less than 1 weightpercent.

The polymer is preferably a substantially homopolymeric polystyrenepolymer. That is, the polymer is preferably derived for a singlemonomer, that monomer being styrene. The phrase "substantiallyhomopolymeric polymer" is herein defined as a polymer in which at least99 percent of the monomeric units (which reacted to form thepolystyrene) were a single monomer. Preferably, at least 99.9 percent ofthe monomeric units which are reacted to form the polymer are a singlemonomer species.

Preferably the polymer is a substantially unsubstituted polymer. Thephrase "substantially unsubstituted polymer" is herein defined as apolymer having a carbon backbone and branches in which less than 2percent of the available sites for substitution have atoms other thanhydrogen thereon. Still more preferably, the degree of substitution isless than 0.5 percent, based on the total number of positions forsubstitution available on the polymer.

A preferred polymer of the present invention exhibits: (I) apolydispersity of from about 1.5 to less than 2.0; (2) a weight averagemolecular weight of from greater than 190.000 to about 250,000; and (3)an M_(z) :M_(n) of from about to 2.5 to about 3.3. Furthermore, thispreferred polymer is branched to from 0 to less than 2 weight percent.Finally, this preferred polymer is a substantially homopolymeric,unsubstituted polymer.

A still more preferred polymer of the present invention exhibits: (1) apolydispersity of from about 1.7 to about 1.98; (2) a weight averagemolecular weight of greater than about 200,000 to about 220,000; and (3)an M_(z) :M_(n) of from about 2.7 to less than about 3.0. Furthermore,this still more preferred polymer is branched to from 0 to less than 1weight percent. Finally, this still more preferred polymer is asubstantially homopolymeric, unsubstituted polymer.

Preferably the formulation of the present invention further comprises achain transfer agent. Chain transfer agents having a transfer constant K(as defined in Vollmert, Grundriss der Makromolekularen Chemie,published by Springer 1962, pages 52 and 71, which is herebyincorporated by reference) of from 0.1 to 50, preferably from 1 to 30,are used. Examples of suitable chain transfer agents are:

    ______________________________________                                        n-Dodecyl mercaptan    (K = 19)                                               tert.-Dodecyl mercaptan                                                                              (K = 3)                                                n-Butyl mercaptan      (K = 22)                                               tert.-Butyl mercaptan  (K = 3.6)                                              Carbon tetrabromide    (K = 2.2)                                              Pentaphenylethane      (K = 2.0)                                              ______________________________________                                    

Optionally (but preferably) the process of the present inventionutilizes a flame retardant in the mixture of components which makes upthe formulation of the expandable beads. In general, the flame retardantis an organic bromine or chloring flame retardant compound present in anamount of from about 0.2 to about 2 weight percent, based on the weightof the total formulation. More preferably the formulation comprises abrominated hydrocarbon flame retardant in an amount of from about 0.5 toabout 1.5 weight percent, based on the weight of the total formulation.Still more preferably the formulation comprises a flame retardant whichis at least one member selected from the group consisting oftrisdibromo-propylphosphate, hexabromocyclododecane and bis allyl etherof tetrabromo-bis-phenol A, wherein the flame retardant is present in anamount of from about 0.6 to about 1.2 weight percent, based on the totalweight of the formulation.

Preferably (but optionally) the formulation further comprises a "flameretardant synergist", i.e. one or more compounds which increase theeffectiveness of the flame retardant when used in combination therewith.The flame retardant synergist may at least one member selected from thegroup consisting of dicumyl peroxide and other organic peroxides whichhave a half-life of one hour at temperatures of from about 110° C. toabout 150° C.

The formulation may further comprise additional additives which impartparticular properties to the expandable products. Examples includeflameproofing agents based on organic bromine or chlorine compounds,e.g. trisdibromopropyl phosphate, hexabromocyclododecane andchloroparaffin as well as synergists for flameproofing agents, such asdicumyl peroxides and other organic peroxides which decompose at hightemperatures, antistatic agents, stabilizers, colorants, lubricants,fillers, substances which prevent agglomeration during prefoaming, e.g.zinc stearate, melamine-formaldehyde condensates or silica, and agentsfor reducing the demolding time during final foaming, e.g. glycerolesters or hydroxycarboxylic acid esters. Depending on their intendedeffect, the additives may be homogeneously dispersed in the particles orbe present as a surface coating.

The unexpanded beads are of course primarily comprised of one or morepolymers having the characteristics described above. Preferably thepolymer is polystyrene or polymers produced by polymerizing monomerswhich are derivatives of polystyrene. In general, the bead is comprisedof polymer in an amount of from about 93 to about 98 weight percent,based on total bead weight. Preferably the bead is comprised of polymerin an amount of from about 94 to about 97.5 weight percent. Still morepreferably the bead is comprised of polymer in an amount of from about95 to 97 percent, and most preferably the bead is comprised of polymerin an amount of about 96 weight percent.

If a crosslinking polyvinyl compound is present, it should not bepresent in an amount which produces an undesirably high amount ofcrosslinking. This is because the polymer will not undergo expansion ifthe degree of crosslinking is too high. The crosslinking agents whichmay be employed in the process of the present invention comprise:divinylbenzene, diethylene glycol dimethacrylate, diisopropenylbenzene,diisopropenyldiphenyl, diallylmaleate, diallylphthalate, allylacrylates,allylmethacrylates, allylfumarates, allyllitaconates, alkyd resin types,butadiene or isoprene polymers, cyclooctadiene, methylene norbornylenes,divinyl phthalates, vinyl isopropenylbenzene, divinyl biphenyl, as wellas any other di- or poly-functional compound known to be of use as acrosslinking agent in polymeric vinyl-addition compositions.

EXAMPLE 1 (Method of Making Polymer)

A mixture of 87 parts of water, 0.16 parts of sodium pyrophosphate, and0.27 parts of magnesiun sulfate heptahydrate was reacted with stirringat ambient temperature in a stainless steel pressure resistant vessel.To this mixture was added a mixture of 100 parts of styrene, 014 partsof benzoyl peroxide, 0.32 parts t-butylperbenzoate, 0.62 parts ofhexabromocyclododeane, and 0.21parts of dicumyl peroxide, with stirring.The vessel was heated for at least 2 hours at a constant rate to 85° C.and then to 115° C. over 4.5 hours. Sixty-five to seventy-five minutesafter the vessel reached 80° C., 2.9 parts of a 10% aqueous solution ofpoly-n-vinylpyrrolidone was added to the reaction mixture. After anadditional 100-120 minutes, a solution of 0.10 parts of chain transferagent in 4.7 parts of n-pentane was added to the reaction vessel. Afterreaching 115° C., the vessel was held at constant temperature for 3hours, whereupon it was cooled to ambient temperature over 3 hours.

EXAMPLE 2

A polystyrene polymer was prepared substantially as described inexample 1. The polymer contained approximately 3.1 percent pentaneblowing agent). The resulting expandable polystyrene beads were analyzedaccording to the procedure of Example 7, and were found to ave containedpolymer having a polydispersity of 1.82, a weight average molecularweight of 202,000, and an Mz:Mn of 2.70. The beads were screened to0.6-1.3 mm diameter, dried to remove surface moisture, and coated with0.12 weight percent of a mixture of powdered lubricants and antilumpingagents commonly used in the industry as screening aids and antilumpingagents. The pentane content of the beads out of the polymerizationreactor was 3.41 weight percent. However, as was typical, about 0.3weight percent of pentane was lost during subsequent processing, making3.1 weight percent the actual pentane content at the time of expansion.

The coated beads were expanded in a Tri Manufacturing Model 502expander. The inlet steam temperature was about 211 .F, and the inletsteam flow rate was approximately 74 pounds per hour. The first-passexpansion rate was about 208 pounds per hour and the outlet density ofthe prepuff was about 1.9 pounds per cubic foot. A fluidized bed drier(as commonly used in the industry) was utilized to cool and partiallystabilize the resulting prepuff. The fluidized bed dryer was equippedwith a blower which fluidized a portion of the beads with ambient air.The prepuff was then pneumatically conveyed to storage bags and aged atambient temperature and humidity for about 3 hours. Following aging, theprepuff was expanded again in the same expander operated at the sameconditions, the result being a prepuff having a density of about 1.10lb/cu.ft., the expander operating at a throughput of about 217 poundsper hour. The resulting prepuff was again passed through the fluidizedbed dryer. After airveying again to storage bags and aging for aboutthree hours, the prepuff was transferred to a Kurtz vacuum block mold(4' × 8' × 34") and molded. The molding cycle consisted of presteamingvacuum to about 0.5 bar absolute pressure, followed by cross-steamingand autoclaving with steam. The resulting block was then cooled withvacuum until the foam pressure was stabilized. The pressure release timewas about 30 seconds, and the resultant block had an average of 10%fusion, 1% shrinkage (defined as actual block length shrinkage 24 hoursafter molding compared to actual mold length), 1.6% collapse (defined asactual thickness shrinkage in middle of block compared to actual moldthickness), and a bulk density of 1.10 lb/cu.ft. (defined as weight ofblock divided by actual mold volume).

EXAMPLE 3

This example illustrates the expansion and molding of relatively largepolystyrene beads containing 4.4 weight percent pentane. The effect ofinsufficient aging time prior to molding is herein demonstrated. Inaddition, a comparison is given with respect to materials containingconventional amounts of pentane. Also shown is the effect of too short aaging time (after the second preexpansion pass but before molding) withrespect to pentane materials containing conventional amounts of pentane(approximately 6 weight percent).

An expandable polystyrene bead product containing an average of 4.40weight percent pentane and a bead diameter of 1.3-1.9 mm was expanded ina Kurtz KV1000 expander equipped with a Kurtz automatic density controlsystem which adjusted inlet steam flow to achieve desired prepuff outletdensity. A fluidized-bed dryer was utilized for both expansions. Thefirst-pass expansion was at a rate of 2000 lbs/hr and a density of1.22-1.25 lb/cu.ft. After about two hours age, the prepuff was expandedagain, at a rate of 3000 lbs/hr to a density of 0.88-0.90 lb/cu.ft. Theprepuff was then molded on a Kurtz vacuum block mold (26" ×49.5" ×196").No pre-steam vacuum was used. Steam was added at a pressure of about 0.6bar for about 6 seconds cross-steam followed by about 10 secondsautoclave. Vacuum was used to cool the block in the mold. After only onehour prepuff aging, the blocks were of poor quality, i.e. poorly fusedand deformed (poor dimensional stability). After 3-4 hours prepuffaging, the blocks were molded to 0.7 bar maximum foam pressure and wereof excellent quality with a total cycle time of 160-170 seconds. Typicalcycle times with normal pentane product after 24-36 hours aging were300-360 seconds.

EXAMPLE 4

This example illustrates how the use of a 4.4% pentane formulationcompares favorably with respect to the use of a conventionalformulation. Expansion and molding results, cycle time, fusion, anddimensional stability were acheived with the 4.4% pentane formulationover a conventional 6% pentane formulation.

An expandable polystyrene bead product containing an average of 4.40%pentane and a bead diameter of 0.6-1.3 mm was expanded in a Weiser VN400expander equipped with a fluidized-bed dryer. The firstpass rate wasabout 2600 lb/hr at an outlet density of about 120 lb/cu.ft. After agingfor about four hours, the prepuff was expanded again, at a rate of about4000 lb/hr and at an outlet density of about 0.79-0.88 lb/cu.ft. Afterabout one hour of aging, the prepuff was molded in a Weiser VacuCompactblock mold (196" ×49" ×31"). Cycle times, fusion, and dimensionalstability were equal to or better than that of products of normal (6%)pentane content which had been aged overnight (i.e. over eight hours ofaging).

EXAMPLE 5

This example illustrates, among other results, the advantageousperformance of a formulation comprising 3.6% pentane. Note the highlydesirable low aging time as well as the desirable molding cycle timewith accompanying high dimensional stability after molding. Anexpandable polystyrene bead product containing an average of 3.58%pentane and a bead diameter of from 0.6-1.3 mm was expanded in a WeiserVN400 expander equipped with fluidized bed drying. The first-passexpansion rate was about 3100 lb/hr at an outlet density o 1.59lb/cu.ft. After aging about four hours, the prepuff was expanded again,at a rate of about 3400 lb/h and outlet density of about 0.84-0.86lb/cu.ft. After about one hour age, the prepuff was molded on a WeiserVacuCompact block mold. Pressure-release (i.e., cooling) time was only29 seconds. All blocks were well fused and dimensionally stable.

EXAMPLE 6

This example illustrates, among other advantages, how a formulationcomprising 3.63 percent pentane permits a highly advantageous moldingcycle time, with accompanying 50% increase in productivity in themolding step, due to the lower molding cycle time. Good fusion anddimensional stability are also shown. An expandable polystyrene beadproduct containing an average of 3.63% pentane and a bead diameter offrom 0.6-1.3 mm was expanded in a Dingledein & Herbert VA-K2000expander. The first-pass expansion rate was 3000 lb/hr at an outletdensity of 1.66-1.75 lb/cu.ft. After aging for about 24 hours, theprepuff was expanded again, at a rate of about 4000 lb/hr and an outletdensity of about 0.92-0.94 lb/cu.ft. After aging for about two hours,the prepuff was molded in a 16' Tri Manufacturing block mold.Well-fused, dimensionally stable blocks were produced at a rate of about15 blocks per hour, as compared to 10 blocks per hour for normalpentane-content beads.

EXAMPLE 7 (Molecular Weight Distribution Curve Determination)

The following equipment and procedure was utilized in order to generatethe molecular weight distribution curve for polystyrene polymers. Thisprocedure was utilized to both determine the molecular weightdistribution of the polystyrene polymer of the present invention, aswell as to analyze products which are herein compared and contrastedwith the polymer and formulation of the present invention.

Chromatography Equipment and Conditions

The apparatus consisted of a Waters 6000A pump with a U6K injector, aViscotek-supplied pulse dampener, two 30 cm PLGel 5 um Mixed Bedpolystyrene columns, a Viscotek Model 100 differential visometer (DV)and a Waters R401 differential refractometer (RI). The data acquisitionand analysis hardware consisted of an IBM PC AT equipped wit 640 kb RAM,a 30 Mb fixed disk and two 5.25" floppy disk drives; a dot matrixprinter and an HP 7475A plotter. The software used was Unical Ver. 3.11(an ASYST-based package) modified to display M_(z+1) and obtained fromViscotek.

The chromatographic conditions were as follows:

    ______________________________________                                        Nominal flow rate:                                                                           1.0 ml/min                                                     Solvent:       THF, high purity, non-spectro grade                            sample Injection Volume:                                                                     0.100 ml                                                       RI Detector:                                                                  Attenuation:   16×                                                      Polarity:      +                                                              DV Detector:                                                                  Temperature:   31.0 ± 0.1° C.                                       Full Scale Output:                                                                           50 Pa                                                          DPT Sensitivity:                                                                              0.2074                                                        Data Acquisition:                                                             Start Time:     6 min.                                                        Stop Time:     24 min.                                                        ______________________________________                                    

Analytical Procedure

The THF to be used as the mobile phase for the GPC system was filteredthrough a 0.45 um fritted filter and then degassed under an aspiratorvacuum for approximately 45 minutes. The THF and the flask were thentransferred to the GPC system and the THF maintained under a pad ofhelium. Samples were made up to a concentration of 5 mg/ml and filteredthrough a Gelman Acrodisc CR PTEE 0.45 um filter prior to injection.

Only freshly prepared solutions were used as polymer degradation wasapparent with aged solutions. All solutions were analyzed twice. The EPS(expanded polystyrene) samples were not purified by precipitation priorto dissolution and analysis, as comparison of purified and raw EPSpolymer results indicated no significant differences. To obtain accuratesolution concentrations for the unprecipitated EPS, the initial solutionconcentrations were corrected for volatiles determined by GC andcoulombmetric analysis for pentane and moisture, respectively, orgravimetrically by baking a sample for total volatiles.

To correct for fluctuations in flow rate, each chromatogram wasnormalized to the flow rate present during calibration. This wasaccomplished by calculating the ratio of the void volumes (totalexclusion volume, earliest negative peak in chromatogram) of thecalibration chromatograms to the sample chromatogram. The calibrationvoid volume was determined to be 19.72 ml. The ratio was then enteredinto the data analysis package as the corrected flow rate.

EXAMPLES 8-15

The procedure of Example 1 was substantially followed in making apolystyrene formulation according to the present invention. This polymerformulation is identified as Example 8 in Table I (infra). The polymerformulation of Example 9 contained a blowing agent (pentane) in anamount of about 3.5 weight percent. The polymer of Example 9 wasanalyzed according to the procedure set forth in Example 7, and fromthis analysis the number average molecular weight (Mn), the weightaverage molecular weight (Mw), and the z-average molecular weight(M_(z)) were determined. From these values the polydispersity (PD) andthe Mz:Mn were calculated. The analytical procedure of Example 7 wasalso performed for several current commercial products (i.e. Examples9-14), each of which contained blowing agent in an amount of 5.5 weightpercent to at least 7 weight percent. Example 10 had blowing agenttherein at a level of approximately 6 weight percent. From thisanalysis, the same molecular weight determinations were made. Table I(below) provides the results of the analyses for both the formulation ofthe present invention (Example 9) as well as several commercialformulations currently available.

As can be seen from Table I, only the formulation of the presentinvention had all three identifying characteristics within the scope ofthose which are identified as pertaining to the polymer of the presentinvention. Even though each of these polymers falls within thedefinition of the polymer utilized in the formulation of the presentinvention, none of these formulations had the amount of blowing agentrequired in the formulation of the present invention.

                  TABLE I                                                         ______________________________________                                        POLYMER CHARACTERISTICS                                                       EX-    M.sub.n × 10E5                                                                    M.sub.w × 10E5                                                                         M.sub.z × 10E5                          AMPLE  (g/mol)   (g/mol)   PD   (g/mol) M.sub.z :M.sub.n                      ______________________________________                                         8     1.12      2.02      1.82 3.00    2.70                                   9     1.12      2.17      1.95 3.35    2.99                                  10     0.87      1.93      2.20 3.11    3.56                                  11     1.37      2.89      2.10 5.28    3.84                                  12     1.20      2.51      2.09 4.58    3.81                                  13     1.01      1.89      1.88 3.07    3.04                                  14     1.13      2.20      1.94 3.83    3.37                                  15     1.27      2.73      2.15 5.03    3.96                                  ______________________________________                                         [polydispersity was calculated as M.sub.w /M.sub.n, and the ratio of          M.sub.z to M.sub.w was calculated by dividing the value obtained for Mz b     the value obtained for Mn                                                

EXAMPLES 16-17

These two examples illustrate the difference in both (1) totalemissions, as well as (2) emissions during aging, for expandedpolystyrene which was produced according to the process described inExample 1. Example 16 illustrates the emissions from a "conventional"process which employs a blowing agent (pentane) in an amount of 6 weightpercent. Example 17 illustrates, in contrast, the emissions from aprocess employing a formulation which comprises only 3.5 weight percentpentane. For Example 16 the amount of blowing agent added during thepolymerization was approximately 6 weight percent, whereas for Example17 only about 3.5 weight percent blowing agent was added during thepolymerization. Table II (infra) provides the results of emissionsduring each of the expansion steps for each Example, for each agingperiod for each Example, and during the molding step for each Example.The bottom row of Table II provides figures for the total emissionsduring the entire process of expansion, aging and molding.

As can be seen from the figures in Table II, the polystyrene having thelow initial level of blowing agent (i.e. Example 17) exhibited a totalemissions of only from 42 to 79 percent as much as for Example 16.Furthermore, the total emissions during aging was only from about 17percent to about 48 percent for Example 17 as compared with Example 16.Accordingly, the formulation (and polymer) of the present inventionexhibit a substantial reduction in both the total emissions as well asthe emissions during aging.

                  TABLE II                                                        ______________________________________                                                      Example 16                                                                             Example 17                                             ______________________________________                                        blowing agent content                                                                         6 wt. percent                                                                            3.5 wt. percent                                    1st Pass Expansion                                                                            .76        .45                                                1st pass aging emissions                                                                      2.0        .12                                                1st pass aging time                                                                           24 hours   4 hours                                            2nd pass Expansion                                                                            N/A        .07                                                2nd pass aging emissions                                                                      N/A        .22                                                1nd pass aging time                                                                           N/A        (2 hours)                                          molding emissions                                                                             0.6-1.1    0.76%                                              Total Emissions  3.36-3.86%                                                                              1.62%                                              ______________________________________                                    

EXAMPLE 18 (Method of Determining and Calculating the Degree ofBranching)

Branching can be determined using a Viscotek differential viscometer andrelated software according to the theory of Zimm and Stockmayer. Moreextensive and descriptive discussions of this theory, as well as relatedsubject matter, can be found in Billmeyer, F. W., Jr., Textbook ofPolymer Science, 2nd Ed., 1971, Wiley-Interscience, N.Y., N.Y.,especially pages 89-90, which is herein incorporated by reference. Usingthe Mark-Houwink constants for known linear samples of polystyrene, onecan calculate the branching frequency of the number of branches per 100monomer units. More extensive and descriptive discussions of theseconstants, as well as related subject matter, can be found in Billmeyer,F. W., Jr., Textbook of Polymer Science, 2nd Ed.,1971,Wiley-Interscience, N.Y., N.Y., especially pages 86-87, which areherein incorporated by reference.

We claim:
 1. A process for making a closed-cell foamed thermoplasticresinous molded object, the process comprising the steps of:A.preexpanding unexpanded beads in from 2 to 4 preexpansion steps, thebeads being comprised of:i. a blowing agent which is at least one memberselected from the group consisting of:pentane, cyclopentane,methylcyclopentane, neopentane, isopentane, pentane petroleum distillatefractions, 2-methyl pentane, 3-methyl pentane, 2,2-dimethylbutane,2,3-dimethylbutane, 2-methyl pentane, 3-methyl pentane, propane, butane,isobutane, hexane, isomers of hexane, cyclohexane, methylcyclohexane,heptane, propylene, butylene, isobutylene, mixtures of one or morealiphatic hydrocarbons having a molecular weight of at least 42 and aboiling point not higher than 95° C. at 760 millimeters absolutepressure, water, carbon dioxide, ammonium carbonate, and azo compoundsthat are decomposable to form a gas at a heat-plastifying temperature towhich the polymer is brought, the blowing agent being present in thebeads in an amount of from about 2 weight percent to about 4.4 weightpercent based on the weight of the beads; and ii. a polymer producedfrom at least one monomer, wherein the monomer is at least one memberselected from the group consisting of:styrene, derivatives of styrene,vinyltoluene, mono- and polyhalogenated vinyltoluenes which form linearpolymers, acrylonitrile, methyl methacrylate, and vinyl toluene, thepolymer being present in the beads in an amount of from about 93 weightpercent to about 98 weight percent based on the weight of the beads, thepolymer exhibiting:(a) a polydispersity of from about 1 to less than2.5, (b) a weight average molecular weight of greater than about 180,000to about 300,000, and (c) an M_(z) :M_(n) of from about 2 to about 4.5,wherein the polystyrene polymer is branched to from 0 to less than 5weight percent, and wherein the preexpansion steps are carried out in anexpander and at substantially atmospheric pressure, and wherein thepreexpansion steps are carried out so that finally-preexpanded beads areproduced, and B. molding the finally-preexpanded beads in order to bothfuse and further expand the finally-preexpanded beads, so that a moldedfoamed object having a density of from about 0.8 pounds per cubic footto about 1.1 pounds per cubic foot is formed.
 2. A method as describedin claim 1 wherein the blowing agent is at least one member selectedfrom the group consisting of pentane, cyclopentane, methylcyclopentane,neopentane, isopentane, pentane petroleum distillate fractions, propane,butane, isobutane, hexane, isomers of hexane, cyclohexane,methylcyclohexane, heptane, propylene, butylene, isobutylene, mixturesof one or more aliphatic hydrocarbons having a molecular weight of atleast 42 and a boiling point not higher than 95° C. at 760 millimetersabsolute pressure, water, carbon dioxide, and ammonium carbonate.
 3. Amethod as described in claim 1 wherein the blowing agent is at least onemember selected from the group consisting of pentane, cyclopentane,neopentane, isopentane, pentane petroleum distillate fractions, propane,butane, isobutane, hexane, cyclohexane, and heptane.
 4. A method asdescribed in claim 1 wherein the blowing agent is present in theunexpanded beads in an amount of from about 2.5 weight percent to about4.4 weight percent.
 5. A method as described in claim 1 wherein theblowing agent is present in an amount of from about 3 weight percent toabout 4 weight percent.
 6. A method as described in claim 1 wherein theblowing agent is present in an amount of about 3.5 weight percent.
 7. Amethod as described in claim 1 wherein the polystyrene polymer isproduced from at least one monomer which is at least one member selectedfrom the group consisting of styrene and derivatives of styrene.
 8. Amethod as described in claim 1 wherein the polymer comprises from about94 to about 97.5 weight percent of the weight of the beads.
 9. A methodas described in claim 1 wherein the polymer comprises from about 95 toabout 97 weight percent of the weight of the beads.
 10. A method asdescribed in claim 1 wherein the polymer comprises from about 96 weightpercent of the weight of the beads.
 11. A method as described in claim 1wherein from 2 to 3 expansion steps are utilized to produce the expandedbeads.
 12. A method as described in claim 1 wherein 2 expansion stepsare utilized to produce the expanded beads.
 13. A method as described inclaim 1 wherein the flow rate of the beads through the expander is fromabout 5 lb/hr/cu.ft. to about 120 lb/hr/cu.ft.
 14. A method as describedin claim 1 wherein the flow rate of the beads through the expander isfrom 7 lb/hr/cu.ft. to about 100 lb/hr/cu.ft.
 15. A method as describedin claim 1 wherein the flow rate of the beads through the expander isfrom about 12 to about 80 lb/hr/cu.ft.
 16. A method as described byclaim 1 wherein the expansion steps are carried out by exposing thebeads to steam.
 17. A method as described by claim 1 wherein the steamhas been heated to a temperature of from about 200° F. to about 220° F.upon entering the expander.
 18. A process as described in claim 1wherein the polydispersity is from about 1.0 to less than 2.0.
 19. Aprocess as described in claim 1 wherein the polydispersity is from about1.5 to less than 2.0.
 20. A process as described in claim 1 wherein thepolydispersity is from about 1.7 to about 1.98.
 21. A process asdescribed in claim 1 wherein the weight average molecular weight is fromgreater than about 190,000 to about 250,000.
 22. A process as describedin claim 1 wherein the weight average molecular weight is from about200,000 to about 220,000.
 23. A process as described in claim 1 whereinthe M_(z) :M_(n) is from about 2.5 to about 3.3.
 24. A process asdescribed in claim 1 wherein the M_(z) :M_(n) is from about 2.7 to about3.0.
 25. A process as described in claim 1 wherein the polystyrenepolymer is branched to from 0 to about 2 weight percent.
 26. A processas described in claim 1 wherein the polystyrene polymer is branched tofrom 0 to about 1 weight percent.
 27. A process as described in claim 1wherein the polystyrene polymer is substantially homopolymeric.
 28. Aprocess as described in claim 1 wherein the polystyrene polymer issubstantially unsubstituted
 29. A process as described in claim 1wherein the formulation further comprises a chain transfer agent.
 30. Aprocess as described in claim 1 wherein the formulation furthercomprises a flame retardant which is an organic bromine or chlorineflame retardant compound present in an amount of from about 0.2 to about2 weight percent, based on the weight of the total formulation.
 31. Aprocess as described in claim 1 wherein the formulation furthercomprises a flame retardant consisting of a brominated hydrocarbon flameretardant, the flame retardant being present in an amount of from about0.5 to about 1.5 weight percent, based on the weight of the totalformulation.
 32. A process as described in claim 1 wherein the flameretardant is at least one member selected from the group consisting oftrisdibromopropyl phosphate, hexabromocyclododecane and bis allyl etherof tetrabromo-bis-phenol A, the flame retardant being present in anamount of from about 0.6 weight percent to about 1.2 weight percent,based on the total weight of the formulation.
 33. A process as describedin claim 1 wherein the formulation further comprises a flame retardantsynergist which is at least one member selected from the groupconsisting of dicumyl peroxide and other organic peroxides which have ahalf-life of 1 hour at temperatures of from about 110° C. to about 150°C.
 34. A method as described in claim 1 wherein from 2 to 3 premoldingexpansion steps are utilized to produce the finally preexpanded beads.35. A method as described in claim 1 wherein 2 premolding expansionsteps are utilized to produce the finally-preexpanded beads.
 36. Amethod as described in claim 1 wherein 2 premolding expansion steps areutilized to produce the finally-preexpanded beads.
 37. A process formaking a molded expanded polystyrene product, the process comprising thesteps of:A. preexpanding unexpanded beads in from 2 to 3 preexpansionsteps, the beads being comprised of:i. a blowing agent which is at leastone member selected from the group consisting of:pentane, cyclopentane,neopentane, isopentane, pentane petroleum distillate fractions, propane,butane, isobutane, hexane, cyclohexane, and heptane, the blowing agentbeing present in the beads in an amount of from about 3 weight percentto about 4 weight percent based on the weight of the beads; and ii. apolymer produced from at least one monomer, wherein the monomer is atleast one member selected from the group consisting of styrene andderivatives of styrene, the polymer being present in the beads in anamount of from about 94.6 weight percent to about 97 weight percentbased on the weight of the beads, the polymer exhibiting:(a) apolydispersity of from about 1.5 to about 2.0, (b) a weight averagemolecular weight of greater than about 190,000 to about 250,000, and (c)an M of from about 2.5 to about 3.3, wherein polystyrene polymer isbranched to from 0 to less than 2 weight percent, and wherein thepreexpansion steps are carried out in an expander and at substantiallyatmospheric pressure, and wherein the preexpansion steps are carried outso that finally-preexpanded beads are produced: and B. molding thefinally-preexpanded beads in order to both fuse and further expand thefinally-preexpanded beads, so that a molded foamed object having adensity of from about 0.8 pounds per cubic foot to about 1.1 pounds percubic foot is formed.
 38. A process for making a molded expandedpolystyrene product, the process comprising:A. preexpanding unexpandedbeads in 2 preexpansion steps, the beads being comprised of:i. a blowingagent which is at least one member selected from the group consisting ofpentane and isomers of pentane, the blowing agent being present in theunexpanded beads in an amount of about 3.5 weight percent, based on theweight of the beads; and ii. a polymer produced from at least onemonomer, wherein the monomer is at least one member selected from thegroup consisting of styrene and derivatives of styrene, the polymerbeing present in the beads in an amount of from about 96 weight percentbased on the weight of the beads, the polymer exhibiting:(i) apolydispersity of from about 1.7 to about 1.98, (ii) a weight averagemolecular weight of greater than about 200,000 to about 220,000, and(iii) an M_(z) M_(n) of from about 2.7 to about 3.0, wherein thepolystyrene polymer is branched to from 0 to less than 1 weight percent,and wherein the preexpansion steps are carried out in an expander and atsubstantially atmospheric pressure, and wherein the preexpansion stepsare carried out so that finally-preexpanded beads are produced; and B.molding the finally-preexpanded beads in order to both fuse and furtherexpand the finally-preexpanded beads, so that a molded expandedpolystyrene product having a density of from about 0.8 pounds per cubicfoot to about 1.1 pounds per cubic foot is formed.